The present invention relates generally to engine compression release braking, and more particularly to a strategy for reducing gas exchange valve seating velocities during engine braking.
The concept of engine compression release braking is well known in the art. In general, engine brakes are designed to open the exhaust valves or a special compression release valve of a internal combustion engine cylinder near the end of its compression stroke. As a result, the work done by the engine in compressing the air within the cylinder is not recovered during the expansion stroke of the piston, but rather is dissipated through the exhaust system of the engine.
Engine compression release brakes were first implemented using a cam to actuate the gas exchange valve at an appropriate timing. While these cam actuated braking systems have observed some success, the industry is driven to produce ever higher braking horsepowers and to introduce variable timing and control into compression release braking events. For instance, U.S. Pat. No. 5,586,531 to Vittorio teaches an engine braking cycle that purportedly achieves higher braking horsepowers through timing control of certain key events during a compression release braking cycle. Other recent innovations include the concept of two cycle engine braking, which is accomplished by performing a. braking event with each upward stroke of a piston. In still another relatively recent innovation, higher braking horsepowers are achieved by so called two event engine braking in which the individual cylinder is briefly opened to the exhaust manifold when the piston is near bottom dead center in order to boost the initial pressure of the cylinder and increase the mass therein. While all of these strategies can conceivably produce substantially higher braking horsepowers, for realistic implementation in an engine, there is a need for electronic control that can produce variable timing of all events independent of crank angle and engine speed.
Thus, there is a trend in the industry to introduce electronically controlled compression release brakes so that braking events can be controlled differently at different operating conditions. This trend finds an analogy in fuel systems for engines that have moved in the direction of permitting electronic control of fuel injection timing and quantity independent of engine speed and crank angle position. Caterpillar, Inc. of Peoria Illinois has observed considerable success in implementing electronically controlled hydraulically actuated fuel injection systems into their engines. It is believed that some of the high speed hydraulic technology developed in relation to fuel injection systems could also find potential application in actuating engine compression release brakes with high speed electronically controlled hydraulics that are independent of engine operating conditions. However, a switch from cam actuated engine brakes to hydraulically actuated engine brakes is not without the introduction of new problems. One such problem relates to limiting valve seating velocities in order to avoid accelerated seat wearing and valve stem fatigue.
Valve seating velocities are generally not a problem in cam actuated systems because the seating velocities are generally controlled by the shape of the cam profile to be generally less than about fifty centimeters per second. In the case of hydraulically actuated engine brakes, other strategies must utilized. One strategy includes the use of flow restrictions or so call xe2x80x9csnubbersxe2x80x9d to slow the movement rate of the exhaust valve member when returning toward its closed position. While a snubber strategy can reduce valve seating velocities at some operating conditions, there are often some operating conditions in which valve seating velocities are still unacceptably high. One source of high seating velocities can be due to residual high pressure in the cylinder when the valve member is moving toward its closed position. Such a circumstance could occur, for example, when the exhaust valve is commanded to close when cylinder pressure is still substantially higher than exhaust manifold pressure. In such a case, the residual pressure acts on the valve in a manner that tends to accelerate the same as it approaches its seated position.
The present invention is directed to these and other problems associated with hydraulically actuated compression release brakes.
A method of engine compression release braking includes an initial step of compressing gas in an engine cylinder. The compression release brake valve is then opened at least in part by fluidly connecting a brake actuator to a source of high pressure actuation fluid. A valve closing timing is then determined that will result in a valve seating velocity that is less than a pre-determined velocity. Finally, the compression release brake valve is closed at the valve closing timing at least in part by fluidly connecting the brake actuator to a low pressure actuation fluid reservoir.
In another aspect, an electronic control module includes a means for determining a valve opening timing for fluidly connecting a brake actuator to a source of high pressure actuation fluid. The module also includes a means for determining a valve closing timing for fluidly connecting the brake actuator to a low pressure actuation fluid reservoir that results in a valve seating velocity that is less than a pre-determined velocity.
In still another aspect, a-hydraulically actuated engine compression release braking system includes a engine compression release brake having a hydraulic brake actuator. A control valve has a first position in which the hydraulic brake actuator is fluidly connected to a source of high pressure fluid, and second position in which the hydraulic brake actuator is fluidly connected to a low pressure actuation fluid reservoir. An electronic control module is in control communication with the control valve and includes a means for determining a valve closing timing that results in a valve seating velocity that is less than a pre-determined velocity.